1. Field of the Invention
The present invention relates to an observing apparatus for a mask and wafer for producing a semiconductor circuit element.
2. Description of the Prior Art
In the manufacture of semiconductor circuit elements a mask image is projected onto a wafer to print said image on said wafer. Prior to said printing the mask should be properly aligned with respect to the wafer. Said alignment is usually achieved by illuminating the mask with a light not actinic to said wafer, illuminating the wafer with the light transmitted by said mask wafer and observing the thus illuminated mask and wafer or the alignment marks provided on said mask and wafer through an observing optical system.
As examples of such aligning methods there are already known so-called manual alignment in which an operator observes the positional relationship of mask and wafer through an observing optical system consisting of a microscope and manually achieves the alignment of mask and wafer, and so-called automatic alignment in which the positional relationship of mask and wafer is photoelectrically determined by observation of alignment marks with an observing optical system containing a photoelectrical scanner and is supplied to a processing circuit which automatically displaces either one of the mask and wafer with respect to the other by means of an x-y-.theta. adjuster. The apparatus of the present invention is applicable to either of these methods.
For printing the mask pattern onto the wafer there are known a contact method in which the mask and wafer are maintained in mutual contact, a proximity method in which the mask and wafer are maintained in close proximity, and a projection method in which the mask pattern is projected onto the wafer. The apparatus of the present invention is applicable to the printer utilizing said projection method.
As the projection optical system to be employed in the printer of projection method, there are known a refractive optical system and a reflective optical system, the latter being considered generally advantageous over the former in terms of chromatic aberration.
A reflective optical system and a printer utilizing such optical system are disclosed in the U.S. Pat. No. 3,748,015 and the German OLS No. 2,410924 in which an observing apparatus is composed of an observing optical system to which the light from the mask and wafer is guided by means of a beam splitter positioned therebetween.
Also the present applicant disclosed, in the Japanese Patent application No. Sho 52-120420 filed on Oct. 6, 1977, a printer utilizing a reflective optical system and provided with an observing system on the light source side with respect to the mask.
The present invention relates to an improvement in the observing apparatus for mask and wafer disclosed in connection with the above-mentioned printer, said improvement being featured in enabling removal of the reflected light from the mask which hinders clear observation.
In the observing optical system disclosed in the above-mentioned German OLS No. 2,410924 the light directly reflected from the mask is not introduced into the observing system and hence poses no problem as a beam splitter is positioned between the mask and wafer to guide the light to the observing optical system, but said reflection from the mask becomes a problem in case the observing system is located on the illuminating source side with respect to the mask as in the above-mentioned patent application filed on Oct. 6, 1977.
More specifically the mask generally composed of a glass substrate provided with an evaporated chromium layer provides a strong reflection while the wafer illuminated with the light transmitted by (passed through) the mask provides a weak reflection. For this reason the brightnesses of mask and wafer in the observing system are significantly different, giving rise to difficulty in observation.
A method of eliminating the light reflected from the mask in a printer with an observing system positioned on the illuminating source side is already disclosed in the U.S. Pat. No. 3,853,398 of the present applicant.
Said method consists of illuminating the mask with a linearly polarized light, providing between the mask and wafer a .lambda./4 plate which changes the light transmitted by the mask, reflected by the wafer and again transmitted by the mask into a linearly polarized state in a direction perpendicular to the polarizing direction of the above-mentioned illuminating light, and providing the observing optical system with a polarizing plate (analyser) to eliminate the light thus directly reflected by the mask, thereby eliminating said light.
The above-mentioned method, however, due to the use of said .lambda./4 plate giving rise to a chromatic aberration, is not suitable for use in a printer utilizing a reflective imaging system not associated with chromatic aberration as disclosed in the present invention.